Recent efforts involve further characterization of a newly discovered mycoplasma (Mycoplasma genitalium) in the urogenital tract of patients with non-gonococcal urethritis. The organism shares some partial antigenic relationship to another pathogenic mycoplasma of man (M. pneumoniae), and this relationship is thought to be mediated by similarities in surface components on the unique terminal attachment structure found in both organisms. Polyclonal and monoclonal antibodies to the P1 attachment protein (165 kd) in virulent M. pneumoniae strains did not react with whole cell or soluble preparations from M. genitalium, indicating the latter organism had a distinct attachment moiety. A specific 143 kd protein has been identified as the possible component in the adherence of M. genitalium to tissue cells. Antibody to both the 165 kd P1 protein and the 143 kd protein have been shown to appear in the serum of animals recovering from experimental infections with each of the two established species. Experimental primate infections with M. genitalium indicated the organism is capable of colonizing the male and female urogenital tract, and in some cases invaded the circulatory system. In more recent collaborative studies, clinical and laboratory evaluation of a case of acute arthritis in a 34 year-old female provides fairly strong circumstantial evidence that M. genitalium might also be involved in extragenital tract disease. Studies on other mollicutes (non-sterol requiring acholeplasmas and helical spiroplasmas) have centered around biochemical and molecular features of these organisms and their possible involvement in pathological syndromes in humans.